1. Field of the Invention
The present invention relates to a current driver and a current driving method for generating a plurality of currents.
2. Prior Art
In order to drive a large-screen display panel in which display elements such as organic EL (electro luminescence) elements and the like are formed, a current driving apparatus capable of generating a plurality of driving currents is needed. Therefore, there have been cases where two separate semiconductor chips each of which includes a current driving apparatus thereon and which are placed adjacent to each other are used to form a current driving apparatus.
In general, characteristics of transistors formed on the semiconductor chips vary between different semiconductor chips. For example, when a plurality of semiconductor chips are provided, even with application of the same voltage to both of a gate of a transistor formed on one of the plurality of semiconductor chips and a gate of a transistor formed on another one of the plurality of semiconductor chips, respective current values of drain currents output from the transistors might differ from each other. Moreover, between semiconductor chips formed using different fabrication processes, variation in characteristics of respective transistors formed on the semiconductor chips is large.
Furthermore, even between transistors formed on a single semiconductor chip, characteristics of the transistors might vary. For example, there is variation in characteristics of a plurality of transistors continuously formed and, therefore, even with application of the same gate voltage to a gate of each of the plurality of transistors, respective current values of drain currents flowing in the transistors are not the same. However, variation in characteristics of transistors located close to each other is small. That is, respective current values of drain currents flowing in a plurality of transistors continuously formed exhibit a certain slope.
Hereinafter, the case where a current driver A and a current driver B formed on separate semiconductor chips, respectively, are placed adjacent to each other to form a current driving apparatus will be described. Each of the current drivers A and B includes a plurality of transistors (for example, driving transistors T104A-1 through T104A-K in FIG. 20) continuously provided so as to be connected in series.
In this case, in each of the current drivers A and B, there is no large difference between current values of respective output current from two of the plurality of transistors formed on the same chip and located adjacent to each other (for example, the driving transistor T104A-1 and the driving transistor T104A-2 in FIG. 20).
However, if a transistor of the current driver A and a transistor of the current driver B are located adjacent to each other, there is a large difference between respective current values of output currents from the adjacent two transistors (for example, the driving transistor T104A-K and the driving transistor T104B-1 in FIG. 20).
As has been described above, as for output currents from the current driving apparatus, current values of output currents around a boundary line between the current driver A and the current driver B are largely different and, therefore, the respective current values of output currents from the current driving apparatus are not uniform (or do not exhibit a certain slope). Therefore, when a display panel is driven using such output currents, a brightness of the display panel largely varies around the boundary line.
To suppress such a larger difference between current values of output currents, a current driving apparatus has been conventionally proposed.
<Known Current Driving Apparatus>
An overall configuration of a known current driving apparatus (having a two-chip configuration) is shown in FIG. 20. The current driving apparatus includes a current driver 20A and 20B.
Respective configurations of the current drivers 20A and 20B of FIG. 20 will be described. Note that the current drivers 20A and 20B have the same configuration and therefore the configuration of the current driver 20A will be representatively described.
The current driving apparatus 20A includes input terminals 101LA and 101RA, bias voltage generation sections 202LA and 202RA, driving transistors T104A-1 through T104A-K, output terminals 105A-1 through 105A-K, and control section 206LA and 206RA.
The input terminals 101LA and 101RA receive a reference current Iref from the outside. Each of the bias voltage generation sections 202LA and 202RA outputs the bias voltage VbiasA having a voltage value corresponding to a current value of the reference current Iref supplied to the input terminals 101LA and 101RA to a gate line G203. Moreover, the relationship (also referred to as current-voltage conversion capability) between the current value of the reference current Iref input to the bias voltage generation sections 202LA and 202RA and the voltage value of the bias voltage VbiasA output from the bias voltage generation sections 202LA and 202RA is adjusted, according to control signals CTa-1 through CTa-P and control signals CTb-1 through CTb-P. Each of the driving transistors T104A-1 through T104A-K is connected between a ground node and an associated one of the output terminals 105A-1 through 105A-K and a gate of each of the driving transistors T104A-1T through T104A-K is connected to the gate line G203A. Thus, output currents Iout-A(1) through Iout-A(K) flow in the driving transistors T104A-1 through T104A-K, respectively. The output terminals 105A-1 through 105A-K output the output currents Iout-A(1) through Iout-A(K) flowing in the driving transistors T104A-1 through T104A-K to the outside. Each of the control sections 206LA and 206RA is turned to be in a stop state or a drive state, according to operation state instruction signals SA-B from the outside. In a stop state, the control section 206LA (or the control section 206RA) does not output the control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P. In a drive state, the control section 206LA (or the control section 206RA) outputs the control signals CTa-1 through CTa-P and control signals CTb-1 through CTb-P, according to a data signal DATA-(K) (or a gate signal DATA-(A1)) to the bias voltage generation section 202LA (or the bias voltage generation section 202RA). The data signal DATA-A(1) corresponds to a current value of the output current Iout-A(1) output from the output terminal 105A-1. The date signal DATA-A(K) corresponds to a current value of the output current Iout-A(K) output from the output terminal 105A-K.
<Internal Configuration of Bias Voltage Generation Section>
The internal configurations of the bias voltage generation sections 202LA and 202RA of FIG. 20 will be described. The bias voltage generation sections 202LA and 202RA have the same internal configuration and therefore the internal configuration of the bias voltage generation section 202LA will be representatively described with reference to FIG. 21.
The bias voltage generation section 202LA includes P voltage generation transistors T110-1 through T110-P, P selection transistors Sa110-1 through Sa110-P, and P selection transistors Sb110-1 through Sb110-P (where P is a natural number).
The control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P are voltages which activate the selection transistors Sa110-1 through Sa110-P and the selection transistors Sb110-1 through Sb110-P (i.e., N-channel transistors), respectively, when being the H level, and negate the selection transistors Sa110-1 through Sa110-P and the selection transistors Sb110-1 through Sb110-P (i.e., N-channel transistors), respectively, when being the L level.
Moreover, the control signals CTa-1 through CTa-P are in one-to-one correspondence with the control signals CTb-1 through CTb-P, and when one control signal is the H level, the other control signal corresponding thereto is the L level.
As described above, the number of voltage generation transistors out of the voltage generation transistors T110-1 through T110-P which serve at the input side of a current mirror circuit (i.e., the number of voltage generation transistors in which a gate and a drain are connected to each other and the reference current Iref flows) is increased/reduced, thereby adjusting the current-voltage conversion capability of the bias voltage generation section.
<Operation>
Next, the operation of the known current driving apparatus (having a two-chip configuration) of FIG. 20 will be described.
[Current Driver 20A]
In the current driver 20A, the control section 206LA receives an operation state instruction signal SA-B instructing “stop” and the control section 206RA receives an operation state instruction signal SA-B instructing “drive”. Thus, the control section 206LA is turned to be in a stop state. On the other hand, the control section 206RA is turned to be in a drive state where the control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P corresponding to the data signal DATA-(K) are output to the bias voltage generation section 202RA.
[Current Driving Apparatus 20B]
In the current driver 20B, the control section 206LB receives an operation state instruction signal SA-B for instructing “drive” and the control section 206RB receives an operation state instruction signal SA-B for instructing “stop”. Thus, the control section 206LB is turned to be in a drive state where the control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P corresponding to the data signal DATA-(K) are output to the bias voltage generator 202LB. On the other hand, the control section 206RB is turned to be in a stop state.
[Driving Processing]
Next, the input terminal 101RA of the current driver 20A receives the reference current Iref.
Next, the bias voltage generation section 202RA outputs the bias voltage VbiasA corresponding to the current value of the reference current Iref supplied to the input terminal 101RA to the gate line G203A. Accordingly, the output currents Iout-A(1) through Iout-A(K) flow in the driving transistors T104A-1 through T104A-K, respectively.
Next, the output terminals 105A-1 through 105A-K output the output currents Iout-A(1) through Iout-A(K) flowing in the driving transistors T104A-1 through T104A-K, respectively.
On the other hand, the input terminal 101LB of the current driver 20B receives the reference current Iref.
Next, the bias voltage generation section 202LB output a bias voltage having a voltage value corresponding to the current value of the reference current Iref supplied to the input terminal 101LB to the gate line G203B. Accordingly, the output currents Iout-B(1) through Iout-B(K) flow in the driving transistors T104B-1 through T104B-K, respectively.
Next, the output terminals 105B-1 through 105B-K output the output currents Iout-B(1) through Iout-B(K) flowing in the driving transistors T104A-1 through T104A-K, respectively.
[Current Value Measurement Processing]
Next, a current value of the output current Iout-A(K) output from the output terminal 105A-K of the current driver 20A is measured. Meanwhile, a current value of the output current Iout-B(1) output from the output terminal 105B-1 of the current driver 20B is measured.
[Characteristic Adjustment Processing]
Next, the bias voltage generation section 202RA receives a data signal DATA-A(K) corresponding to a measured current value of the output current Iout-A(K). Thus, the current-voltage conversion capability of the bias voltage generation section 202RA is adjusted, so that current values of the output currents Iout-A(1) through Iout-A(K) are changed.
The bias voltage generator 202LB receives a data signal DATA-B(1) corresponding to a current value of a measured output current Iout-B(1). Thus, the current-voltage conversion capability of the bias voltage generator 202LB is adjusted, so that current values of the output currents Iout-B(1) through Iout-B(K) are changed.
As described above, with the bias voltage generation sections 202LA and 202RA (or 202LB and 202RB) provided at first and last ones of the driving transistors T104A-1 through T104A-K (or T104B-1 through T104B-K), respectively, the output currents Iout-A(1) through Iout-A(K) (or Iout-B(1) through Iout-B(K)) can be adjusted. Moreover, the control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P corresponding to the current value of the output current Iout-A(K) are supplied to the bias voltage generation section 202RA. Accordingly, a current value of the output current Iout-A(K) can be set to be a proper value. On the other hand, the control signals CTa-1 through CTa-P and the control signals CTb-1 through CTb-P corresponding to a current value of the output current Iout-B(1) are supplied to the bias voltage generator 202LB and the current value of the output current Iout-B(1) can be set to be a proper value. Thus, current values of output currents Iout-A(K) and Iout-B(1) located closest to the boundary line between the current driver 20A and the current driver 20B can be made to match each other.
However, in the known current driver 20A of FIG. 20, the input terminal 101LA, the bias voltage generation section 202LA and the control section 206LA are unnecessary. In the known current driver 20B of FIG. 20, the input terminal 101RB, the bias voltage generator 202RB and the control section 206RB are unnecessary. In each of the known current drivers 20A and 20B, a component(s) which is unnecessary in an operation has to be provided and therefore a circuit size of the current driver is increased.